Videos
The seminar starts every Thursday at 17:00 CEST (Central European Summer Time) and is livestreamed on our youtube channel.
Once the live stream has finished, we will make all video recordings and .pdf versions of the presentation slides available here.
Thu 20200702 17:00 CEST
Quantum Science Seminar #11: Nanophotonics
Humboldt University
Berlin — Germany
Berlin — Germany
Revisiting LightMatter Interaction in Quantum Nanophotonics
The interaction of a singlemode light field with a
single atom or an ensemble of atoms can be described
by a simple Hamiltonian and has been extensively
studied. Nonetheless, the vector properties of light
in conjunctions with the multilevel structure of real
atoms and their collective response result in rich and
surprising physics. In our group, we investigate this
subject matter using nanophotonic components, such as
subwavelengthdiameter optical fibers and
whisperinggallerymode resonators, for interfacing
light and atoms. I will present three effects that we
observed in experiments with these systems and that go
beyond the standard description of lightmatter
coupling. First, transversally confined light can
locally carry transverse spin angular momentum, which
leads to propagation directiondependent emission and
absorption of light. Second, when imaging an
elliptically polarized emitter with a perfectly
focused, aberrationfree imaging system, its apparent
position differs from the actual position. Third, an
ensemble of atoms can change the photons statistics of
light transmitted through the ensemble. There,
depending on the number of coupled atoms, a
collectively enhanced nonlinearity leads to pronounced
photon bunching or antibunching.
Thu 20200625 17:00 CEST
Quantum Science Seminar #10: Ultrafast Science
Lund University
Lund — Sweden
Lund — Sweden
Atomic photoionization using attosecond pulses
Since the beginning of the millennium, physicists know
how to generate pulses of light of attosecond duration
[1], thus gaining access to this incredibly short time
scale. In this presentation, we will show how
attosecond pulses bring new light on ultrafast
electron dynamics in atomic photoionization. We use
attosecond pulse trains together with a weak infrared
probe to measure both amplitude and phase of
photoionization matrix elements. Our method, which
combines high temporal and spectral resolution [2],
allows us to gain new insights on photoionization
dynamics, including electron correlation and spin flip
induced by spinorbit interaction. In another
experiment, we characterize an electron wavepacket
near an autoionizing resonance in helium using a
Wigner representation [3], and retrieve the
corresponding timedependent density matrix.
References
Thu 20200618 17:00 CEST
Quantum Science Seminar #09: Molecules
JILA, NIST and University of Colorado
Boulder — Colorado — U.S.A.
Boulder — Colorado — U.S.A.
A Fermi gas of polar molecules from 3D to 2D
Quantum degenerate gases of polar molecules provide a
new platform for quantum science [1]. A Fermi gas of
KRb molecules is fully thermalized with atommolecule
interactions and characterized using thermometry based
on suppressed density fluctuations [2]. To demonstrate
the full potential of strong dipolar interactions in
the molecular gas, we apply external electric fields
to explore the exciting interplay between molecular
interaction dynamics and dissipation. By confining KRb
to two dimensional optical traps with a perpendicular
electric field [3], we demonstrate greatly enhanced
elastic collisions with strong suppression of
inelastic loss, with their ratio reaching 100. The
favorable 2D dipolar interactions have led to rapid
thermalization and evaporation of molecules.
References
Thu 20200611 17:00 CEST
Quantum Science Seminar #08: Quantum Transport
University of Geneva
Geneva — Switzerland
Geneva — Switzerland
Quantum transport, low dimensions and cold atomic systems
Measuring the transport properties of a system
connected to reservoirs is one of the most common and
most useful probe of the properties of a solid.
Besides its practical interest transport in quantum
systems poses fundamental and challenging theoretical
questions, since it is one of the simplest
realizations of an out of equilibrium phenomenon. I
will review these issues, in particular in the case of
one and quasione (e.g. ladders) systems. In such
systems we know that interactions lead to unusual
ground states [12] and remarkable properties such as
spincharge decoupling, which of course has strong
consequences for transport properties, in particular
decoupling charge and spin transport [3]. I will
connect these theoretical questions with experiments
done in the context of cold atomic systems that
provide novel ways to probe such physics [4].
References

Quantum Physics in One DimensionOxford University Press20039780198525004

Interactions in Quantum FluidsLecture Notes of the Les Houches Summer School913962011

Spin transport in a onedimensional quantum wirePhysical Review Research20230622020

Band and correlated insulators of cold fermions in a mesoscopic latticePhysical Review X80110532018
Thu 20200528 17:00 CEST
Quantum Science Seminar #07: Photonics
University of Utrecht
Utrecht — Netherlands
Utrecht — Netherlands
Quantum Fractals
The human fascination for fractals dates back to the
time of Christ, when structures known nowadays as a
Sierpinski gasket were used in decorative art in
churches. Nonetheless, it was only in the last century
that mathematicians faced the difficult task of
classifying these structures. In the 80’s and 90’s,
the foundational work of Mandelbrot triggered enormous
activity in the field. The focus was on understanding
how a particle diffuses in a fractal structure.
However, those were classical fractals. This
century, the task is to understand quantum
fractals. Last year, we experimentally realized a
Sierpinski gasket using a scanning tunneling
microscope to pattern adsorbates on top of Cu(111) and
showed that the wavefunction describing electrons in a
Sierpinski gasket fractal has the Hausdorff dimension
d = 1.58 [1,2,3]. However, STM
techniques can only describe equilibrium
properties.
Now, we went a step beyond and using stateoftheart photonics experiments, we unveiled the quantum dynamics in fractals. By injecting photons in waveguide arrays arranged in a fractal shape, we were able to follow their motion and understand their quantum dynamics with unprecedented detail. We built and investigated 3 types of fractal structures to reveal not only the influence of different Hausdorff dimension, but also of geometry [4]. Finally, I will tell you about the dynamics of systems governed by a fractional Langevin equation. It turns out that this kind of approach may describe the Gardner phase in glasses, which is a phase exhibiting a fractal structure in the free energy landscape. We find an anomalous diffusion and reveal the existence of a novel regime, characterizing a Time Glass [5].
Now, we went a step beyond and using stateoftheart photonics experiments, we unveiled the quantum dynamics in fractals. By injecting photons in waveguide arrays arranged in a fractal shape, we were able to follow their motion and understand their quantum dynamics with unprecedented detail. We built and investigated 3 types of fractal structures to reveal not only the influence of different Hausdorff dimension, but also of geometry [4]. Finally, I will tell you about the dynamics of systems governed by a fractional Langevin equation. It turns out that this kind of approach may describe the Gardner phase in glasses, which is a phase exhibiting a fractal structure in the free energy landscape. We find an anomalous diffusion and reveal the existence of a novel regime, characterizing a Time Glass [5].
References

Design and characterization of electrons in a fractal geometryNature Physics151272019

Quantum corral herds surface electrons into a fractal latticePhysics Today72142019

Scientists Trapped Electrons In a Quantum Fractal (And It's Wild!)Youtube2019

Shining light on quantum transport in fractal networks2020

Time Glass: a fractional calculus approach2020
Thu 20200521 17:00 CEST
Quantum Science Seminar #06: Molecules
Harvard University
Cambridge — Massachusetts — U.S.A.
Cambridge — Massachusetts — U.S.A.
Combining Chemistry and Physics in Ultracold Polar Molecules
Advances in quantum manipulation of molecules bring
unique opportunities, including the use of molecules
to search for new physics, harnessing molecular
resources for quantum engineering, and exploring
chemical reactions in the ultralow temperature
regime. In this talk, I focus on the latter topic
where we work toward a detailed microscopic picture of
molecules transforming from one species to another and
reveal several surprises along the way. By preparing
quantumstateselected KRb molecules at a temperature
of 500 nK, we observed reactions proceeding through a
longlived intermediate, which provides a handle to
steer with light the reaction pathway away from its
natural course. Despite the long lifetime that might
allow thermalization, our measurements indicate that
ergodicity does not hold for all degrees of freedom.
References

Ultracold chemistry: No longer a disappearing actPhysics Today73122020

Steering ultracold reactions through longlived transient intermediatesarXiv2002.051402020

Forming a single molecule by magnetoassociation in an optical tweezerarXiv2003.078502020

Dipolar exchange quantum logic gate with polar moleculesChemical Science968302018
Thu 20200514 17:00 CEST
Quantum Science Seminar #05: Quantum Optics
Aarhus University
Aarhus — Denmark
Aarhus — Denmark
Quantum interactions with radiation that moves
How does a quantum system interact with a travelling
pulse of quantum radiation, prepared, e.g., in a
number state or a coherent state of light? You may
think that this problem has been text book material
for decades along with detailed solutions for the case
of simple, few level systems. But, in fact, it has
not. While crucial for multiple effects in quantum
optics and for the entire concept of flying and
stationary qubits, quantum optics textbooks do not
provide a formal description applicable to this
foundational and elementary interaction process. After
the introduction of a new (and simple) theoretical
formalism that, accounts for the interaction of
travelling pulses of quantized radiation with a local
quantum system, I shall discuss applications of the
theory to quantum pulses of optical, microwave and
acoustic excitations and show examples of relevance to
recent experiments with qubits and nonlinear
resonators.
References

Jaynes–Cummings model — Schrödinger picture dynamicsWikipedia20200510

InputOutput Theory with Quantum PulsesPhysical Review Letters1231236042019

Quantum interactions with pulses of radiationarXiv2003.045732020

Scattering into onedimensional waveguides from a coherentlydriven quantumoptical systemQuantum2692018
Thu 20200507 17:00 CEST
Quantum Science Seminar #04: Polaritons
Center for Nanoscience and Nanotechnology
C2N — Université Paris Saclay — CNRS
Palaiseau — France
C2N — Université Paris Saclay — CNRS
Palaiseau — France
Quantum fluids of light in semiconductor lattices
When confining photons in semiconductor lattices, it
is possible to strongly modify their physical
properties and explore the physics of a variety of
Hamiltonians. Photons can behave as finite or even
infinite mass particles, photons can propagate along
topological edge states without back scattering,
photons can become superfluid and behave as massive
interacting particles. These are just a few examples
of exotic properties that we can imprint into quantum
fluids of light in semiconductor lattices. Such
manipulation of light presents not only potential for
applications in photonics, but great promise for
fundamental studies of driven dissipative systems.
After a detailed introduction to quantum fluids of
light, I will illustrate the variety of physical
systems we can emulate with this photonic platform by
presenting some recent experiments related to
quasicrystals, helical photons, and photonic
graphene. Perspectives in terms of quantum
correlations will be discussed.
Thu 20200430 17:00 CEST
Quantum Science Seminar #03: Quantum Optics
Institute of Photonic Sciences (ICFO)
Barcelona — Spain
Barcelona — Spain
The maximum refractive index of an atomic medium
It is interesting to observe that all optical
materials with a positive refractive index have a
value of index that is of order unity. Surprisingly,
though, a deep understanding of the mechanisms behind
this universal behavior seems to be lacking. Moreover,
this observation is difficult to reconcile with the
fact that a single, isolated atom is known to have a
giant optical response, with a resonant scattering
cross section that far exceeds its physical size.
Here, we theoretically investigate the evolution of the optical properties of an atomic ensemble as a function of increasing density, including the effects of multiple scattering and nearfield interactions. We find that the index does not grow indefinitely with density, but rather reaches a limiting value of n ~ 1.7. Using strongdisorder renormalization group theory, we show that this maximum value arises from the combination of random atomic positions and nearfield interactions, which results in a inhomogeneous broadening of atomic resonance frequencies. Thus, regardless of the physical atomic density, light at any given frequency only interacts with approximately one nearresonant atom per cubic wavelength, limiting the maximum index attainable. Finally, we discuss how this simple atomic physics limit might be extended to arrive at a theory for reallife solids.
Here, we theoretically investigate the evolution of the optical properties of an atomic ensemble as a function of increasing density, including the effects of multiple scattering and nearfield interactions. We find that the index does not grow indefinitely with density, but rather reaches a limiting value of n ~ 1.7. Using strongdisorder renormalization group theory, we show that this maximum value arises from the combination of random atomic positions and nearfield interactions, which results in a inhomogeneous broadening of atomic resonance frequencies. Thus, regardless of the physical atomic density, light at any given frequency only interacts with approximately one nearresonant atom per cubic wavelength, limiting the maximum index attainable. Finally, we discuss how this simple atomic physics limit might be extended to arrive at a theory for reallife solids.
Thu 20200423 17:00 CEST
Quantum Science Seminar #02: Quantum Computing
Weizmann Institute of Science
Rehovot — Israel
Rehovot — Israel
Trappedion quantum computing: a coherent control problem
Several systems have been investigated in the last
couple of decades as possible platforms for the
realization of a quantum computer. Among the different
systems examined, trappedion systems have thus far
demonstrated the highest fidelity quantum gates and
very long coherence times. However, scaling
trappedion quantum computers to large numbers of
qubits has proven to be a difficult problem. In this
talk I will review the quantum toolbox of trappedion
quantum computing and discuss the coherent control
techniques that were developed in recent years that
render trappedion quantum gates robust against errors
and allow for quantum computing on long chains of
ions.
Thu 20200416 17:00 CEST
Quantum Science Seminar #01: Quantum Simulation
MaxPlanckInstitute of Quantum Optics (MPQ)
Garching b. München — Germany
Garching b. München — Germany
Analog Quantum Simulation: from physics to chemistry
Manybody systems are
very hard to simulate due to the explosion of parameters with the
system size. Quantum computers can help in this task, although one may
need scalable systems, something that is out of reach in the short
run. An attractive alternative is provided by analog quantum
simulators which, even though they are not universal, they can still
be tuned to study interesting problems. Atoms in optical lattices seem
to be ideally suited for that task. Most of the proposals of such
simulators have focused so far on condensed matter or high energy
physics problems. In this talk I will show how one can extend the
range of problems to other scenarios, especially to quantum chemistry.